I have found that some of the properties in the Report Builder for table columns do not work properly. This is very frustrating, because what is rendered in the Report Builder is not the same as what is rendered on the server when the report is executed. What I have found to work is to create an expression in the value of the cell to minimize the column width.
Say you have a field with a lot of data and it is separated by a special character. This will cause the column to render one line because there is no separation in the data.
Place this code in the value expression of the Text Box Property.
=Replace(Fields!MyField.Value, ";", Environment.NewLine)
Change the ";" character to modify the separator that is present in your data. This will replace each special character with a new line, therefore, narrowing the table column width and giving your rendered report a better look.